The electrophotography development method is a method achieved by adhering toner particles contained in a developer to an electrostatic latent image formed on a photoreceptor. The developers used in this method are classified into two-component developers consisting of toner particles and carrier particles and one-component developers consisting of only toner particles.
Among the developers described above, the cascade method were employed in the past as a development method which uses the two-component developer consisting of toner particles and carrier particles, but the current mainstream is a magnetic brush method using a magnet roll.
In the two-component developer, the carrier particle serves as a carrier material to form a toner image on a photoreceptor by being stirred with toner particles in a development box filled with a developer, imparting the intended charge to the toner particles, and further transferring the thus charged toner particles to the surface of the photoreceptor. The carrier particles remained on a developing roll having a magnet return again to the developing box from the developing roll, and are mixed and stirred with new toner particles for repeated use for a certain period of time.
In two-component developers, unlike one-component developers, carrier particles are mixed and stirred with toner particles to charge the toner particles and further functions to transfer the toner particles, thereby providing good controllability when designing a developer. Consequently, the two-component developer is suitable for full color developing apparatuses which require high image quality and apparatuses used for high-speed printing which require reliability and endurance in image maintenance, etc.
In two-component developers used in such a manner, image properties such as image density, fogging, white spot, gradation, resolution, etc., need to exhibit a determined value from the initial stage. Further, these properties must not fluctuate during toner life and need to be stably maintained. To stably maintain these properties, it is essential that the properties of the carrier particle contained in a two-component developer be stable.
Conventionally, an iron powder carrier, such as iron powder having the surface thereof coated with an oxide film or iron powder having the surface thereof coated with a resin, has been used for the carrier particle forming a two-component developer. These iron powder carriers have high magnetization and high conductivity, thereby being advantageous in that an image with good solid portion reproducibility is easily provided.
However, such an iron powder carrier has a heavy true specific gravity of about 7.8 and magnetization which is also too high. Consequently, the toner components tend to fuse on the surface of iron powder carriers, so-called “toner spent”, from the stirring and mixing with the toner particles in a developing box. The occurrence of such a toner spent decreases the effective carrier surface area, causing the frictional chargeability with the toner particles to be deteriorated.
In a resin-coated iron powder carrier, the resin on the surface may peel off due to the stress during use, exposing the core material (iron powder) having a low breakdown voltage owing to a high conductance, whereby charge may be leaked. The electrostatic latent image formed on the photoreceptor breaks down from such a charge leakage, thus causing brush strokes or the like to occur on the solid portions, making it difficult to produce a uniform image. For these reasons, iron powder carriers, such as an oxide film coated iron powder or a resin coated iron powder, are currently no longer used.
Recently, a ferrite having a light true specific gravity of about 5.0 and low magnetization is used as carrier in place of the iron powder carriers, and a resin coated ferrite carrier coated with a resin on the surface thereof is increasingly used, whereby the developer life has been remarkably extended.
The method for producing such a ferrite carrier is typically carried out by mixing a determined amount of ferrite carrier raw materials, subsequently calcining, crushing and granulating, followed by sintering. The calcining may not have to be performed under a certain condition.
Incidentally, considering the recent more strict environmental regulations, metals such as Ni, Cu, Zn, etc., are now remotely used, and the use of metals in compliance with the environmental regulations are demanded. Accordingly, the ferrite constituent compositions used as the carrier core material have shifted from Cu—Zn ferrite and Ni—Zn ferrite to Mn ferrite, Mn—Mg—Sr ferrite, etc., which use Mn.
Japanese Patent Laid-Open No. 2006-337828 describes a ferrite carrier core material for an electrophotography whose surface is divided into 2 to 50 sections with grooves or lines per 10 μm square and which contains manganese ferrite as a principle component. Such a ferrite carrier core material has a uniform composition, constant surface properties, good fluidity, high magnetization and low resistivity. The electrophotographic developer using the ferrite carrier having such a ferrite carrier core material coated with a resin exhibits a quick charge rise property and is hence thought to have a stable charge level for long time use.
To produce the ferrite carrier core material as described above, Japanese Patent Laid-Open No. 2006-337828 discloses a production method in which a compound oxide containing as principle components Fe and Mn in an Fe to Mn molar ratio (Fe/Mn) of 4 to 16 is pulverized, mixed, granulated and sintered, and further crushed and classified, wherein the sintering is carried out under an atmosphere having an oxygen concentration of 5% by volume or less.
However, Mn is becoming a subject of various laws and regulations. In response to this movement, new carrier core materials free of not only various heavy metals described above but also Mn are demanded.
As an alternative to the carrier core material containing Mn, carrier core materials containing Mg are proposed. For example, Japanese Patent Laid-Open No. 2005-162597 discloses an Mg ferrite material (carrier core material) represented by the formula XaMgbFecCadOe (wherein X represents Li, Na, Ti, etc., or a combination thereof) having a saturated magnetization of 30 to 80 emu/g and a breakdown voltage of 1.5 to 5.0 kV, and that this Mg ferrite material can meet the demands in high image quality and environmental regulations.
Japanese Domestic Re-Publication of PCT Publication No. 2006-524627 discloses an Mg ferrite material (carrier core material) represented by the formula MgaFebCacOd having a saturated magnetization of 30 to 80 emu/g and a breakdown voltage of 1.5 to 5.0 kV, composed of clean materials in compliance with the environmental regulations, providing high image quality that is clear, enhanced in tone and free of fogging.
Despite the proposal of the carrier core material containing Mg as described above, the compatibility of the properties between high magnetization and medium to high resistivity is hard to achieve since the magnetization and resistivity are generally in the trade-off relationship. For this reason, Mn is added to modify the trade-off relationship between the magnetization and resistivity and attain high magnetization with medium to high resistivity, and such an Mn-added core material is thus currently used as the carrier core material for electrophotographic developers. However, as described earlier, the situation is changing against the Mn use as the regulations on various heavy metals are reinforced.
Even with the Mg carrier core material to which Mn is not intentionally added, there is a method for achieving high magnetization and medium to high resistivity by the conventional sintering. That is, the attempt has been made in which the resistivity is adjusted to a desired level by oxidizing the surface after the final sintering, but it is still not enough to solve the trade-off relationship described above.
Further, it is conventionally known that the magnetization can be raised by producing the Mg ferrite using an excessive amount of Fe. However, as a result of the excessive amount of Fe, the resistivity is extremely low. Furthermore, the Fe-excessive Mg ferrite is characterized by the sudden decrease of magnetization caused by the surface oxidization or a high oxygen concentration at the final sintering. The oxidization of divalent Fe contained in the magnetite is considered to be responsible for this phenomenon.
On the other hand, the sintering temperature for the Mg ferrites which do not contain a transition metal other than Fe is as high as about 1250 to 1350° C. Further, the only obtainable surface property required for the carrier core material is a surface with little ruggedness, and many non-spherical particles are contained since the carrier core material particles tend to coagulate each other during the sintering. As a result, a carrier core material for an electrophotographic developer, which is intentionally free of heavy metals, has high magnetization, medium to high resistivity and the surface properties having the right degree of ruggedness with uniform topography, is not yet obtained at present.